With the Deck 82, you can press a maximum of 6 keys at the same time + all the modifier keys (alt's and shift's). This is because that is the limit of the USB spec and is the same for any other USB keyboard on the market.

With the Deck 105, if you chose to go with the PS2 interface, theoretically you could press all the keys at the same time if you like. The PS2 spec allows for more keys to be pressed simultaneously. Also, if you choose the USB interface you would get the same number as the Deck 82 - 6 keys + modifier keys.